Cardiac rupture and aneurysm formation following acute myocardial infarction may be related to degradation of structurally important collagen molecules by inflammatory cell proteolytic enzymes. Using a rat mode of acute myocardial infarction, significant collagen degradation was found to occur in the first twenty-four hours following experimental infarction. this degradation was prevented by eliminating inflammatory cells by prior whole body irradiation, or by treatment with a potent anti-inflammatory agen (ibuprofen). Continued treatment with ibuprofen resulted in enhancement of collagen deposition in the infarct wall.